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Emissivity: Understanding How it Affects Your Thermal Images

I often joke that were it not for emissivity, you’d not need training to use an imager! In our courses we find emissivity is the #1 most confusing issue for people, whether they are engineers, home inspectors or new thermographers. In fact, we all usually have a good laugh that many cannot even pronounce the word. So if you also feel this way about the term or the concept, you are in good company!

Emissivity describes how to quantify the efficiency of a surface for radiating energy in a defined waveband and at a given temperature. Reality says any surface above absolute zero will always radiate some energy (more than 0%), and no surface can radiate perfectly (100%).

When considering infrared radiation most shiny metals emit inefficiently. This means they don’t tell us the thermal truth about themselves! Most non-metal surfaces—paint, paper and human skin, for example—are much more efficient emitters, so it is easy to make a direct connection between what they radiate and their surface temperature. Remember the hot frying pan?

Emissivity values can be determined or measured by engineers. Be aware, however, they are very specific to the material type, surface condition and, especially for metals, the temperature of the material. We can use the values not only to help us understand how a surface might behave but also, in some cases, to correct our radiometric measurements.

A single image often contains many surfaces each with a different emissivity and, as is the case here, with different temperatures. Making accurate corrections for high-emissivity surfaces, like the hand, mug and the water, is fairly straightforward, but the low-emissivity metal spoon is very challenging to work with—more on that next week!

Thinking of these values as percentages may help. Human skin, with a value of 0.98, is 98% efficient at emitting thermal radiation while shiny aluminum, with a value of approximately 0.10, emits only 10% of the energy. When we input these values into our imagers, they automatically correct the raw data that had assumed 100% radiation was emitted based on the surface temperature.

As you can imagine measurements using extreme corrections, as are necessary for bare metals, often are unreliable and that is why we strongly recommend making measurements only on surfaces with values greater than approximately 0.6. On metals, the simplest way is to add a high-emissivity “target” of paint or electrical tape.

When you input a correction, the resulting changes are made to the entire image. Obviously, because of this fact, separate corrections must be made for each different point we want to measure. While some models allow these corrections to be made in the imager itself, the good news is they can also be made in the software to a stored image. All changes can be undone or “tweaked” to better match reality.

I’d encourage you to practice making measurements on various surfaces that are at least 10oC (18oF) warmer or colder than the surroundings—windows, coffee cups, skin, etc. Compare what you measure on the surface with what you measure on a high-emissivity target (use electrical tape with an emissivity correction of 0.94). Notice, too, what happens as you adjust the emissivity correction for these surfaces—the image doesn’t change, but the corrected temperature values do! Next week we’ll talk about what is being reflected from the surface and how to correct for that.

Don’t expect everything to become crystal clear immediately, but you should quickly find out emissivity is not as confusing as it may have seemed to be. Here are two good guidelines:

  • Radiometric temperatures of bright metal surfaces will be unreliable. Use high-emissivity targets whenever possible.
  • Radiometric temperatures of nearly all other surfaces in nearly all instances will be quite reliable.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

5 comments to Emissivity: Understanding How it Affects Your Thermal Images

  • Ben Duffey

    John, thank you for this article, with your approval I would like to use this to educate people involved in, but not necessarily going to training in thermography.
    I have seen many reports where the low emissivity of the object result in incorrect statements made. One would normally think that the more shiny the material, the lower the emissivity but on aluminium this is not always the case.
    In our company there is a lot of IR spot temperature measuring tools used but the people who use them don’t understand emissivity at all. I usually try to explain the importance of emissivity as well as the distance from the object to them, but not all of them believe you.
    The problem sometimes start with the salesperson selling the equipment to them, telling them they can measure everything accurately without explaining the limitations.

  • tepings

    john,thank you for this article,it is very useful for me .But I want more information about emissivity of different thing in one picture .How can I contact with you.

  • Fluke Thermography

    The method to change emissivity of entire images or only select points in an image depends upon the imager being used and the software being used.

  • John, You and Rob Spring wrote an interesting white paper a while back; “A Unique Thermal Problem Found in Certain Double-Glazed Windows”. Unfortunately, there was no mention of the emissivity setting(s) used (or any other methods employed) to effectively capture the anomalies in the windows themselves while minimizing reflected radiation from the images. For this application, I’m more concerned with capturing temperature differences than I am with getting accurate temperature measurements. Can you offer some guidance? Thanks, -Phil

  • Fluke Thermography

    Hi Phil,

    If you don’t really care about temperature calculations, don’t worry about the setting for emissivity. I usually just set it to .95 and leave it.

    Just be aware of reflections and you will usually be able to discern true thermal differences through comparison.

    Best,
    Fluke Thermography

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